1. Field of the Invention
This invention relates to a disc substrate of synthetic resin constituting a disc operating as a recording medium for information signals, such as a magnetic disc, an optical disc or a magneto-optical disc, and a method and apparatus for manufacturing the disc substrate. More particularly, it relates to a disc substrate of synthetic resin in which a head is mounted on a floating slider, with the head and the disc being arranged in close proximity to each other for realizing high-density recording/reproduction.
2. Related Art
In a disc, such as an optical disc, magneto-optical disc or a magnetic disc, a disc substrate is used which is molded from a synthetic resin material exhibiting light transmitting performance, such as polycarbonate resin. This disc substrate has an aperture at a mid portion thereof.
Usually, disc substrates are manufactured by injection molding. That is, in manufacturing the substrates, a substrate material, usually synthetic resin, is melted on heating, and injected under elevated pressure into a cavity corresponding in shape to the desired substrate shape. After cooling and curing of the substrate material, it is taken out from the cavity to manufacture the substrate of a desired shape.
Meanwhile, in usual injection molding, the inside of a metal mold is degassed.
However, since there are interstices in the outer and inner portions of the disc substrate molded in the metal mold, the resin is pulled by this degassing into these interstices to form burrs on the outer and inner portions. These burrs are extremely brittle and ready to break under the effect of various operations performed when taking out the disc substrate out of the metal mold. That is, the disc substrate is significantly lowered in strength in the outer and inner portions thereof due to the burrs formed thereon.
Thus, the disc substrate molded by injection molding experiences a problem that if, during transport of the disc substrate during inspection or assembling steps, a hand area of a transporting jig of the outer rim clamp or outer and/or inner rim vacuum handling system is contacted with the burrs on the outer and/or inner rims, these burrs are detached and turned into projection-like defect of the magnetic film thus lowering the quality of the disc substrate.
Moreover, if, after sputtering the magnetic layer on the disc substrate formed of synthetic resin, pressure is applied to the outer or inner rims of the disc substrate, the burrs on these outer or inner rims, if present, are broken and cracked, thus causing exfoliation due to absorbed or adsorbed water to lower the product quality.
The disc substrate formed of resin by injection molding suffers not only from the burrs, but from the so-called honing phenomenon in which the disc substrate is reduced in thickness as the outermost rim thereof is approached, as shown in FIG. 1. In the hard disc, a floating head is floated by approximately 50 mm relative to the disc surface and is moved substantially radially of the disc. Since the outer rim portion of the disc is of increased recording area and hence the recording area is preferentially set to as close to the outermost rim of the disc for increasing the recording capacity of the disc. Thus, the planar outer rim portion of the disc is desired for preventing the collision between the disc and the floating slider. Therefore, a smaller magnitude of the honing is desirable. In addition, from the convenience in designing the hard disc drive, this honing needs to be suppressed to within a prescribed magnitude. However, the honing tends to exceed the prescribed magnitude if burrs are formed on the disc substrate.